This invention relates to a method and apparatus for adjusting the positive airway pressure of a patient to an optimum value in the treatment of obstructive sleep apnea, and more particularly to a breathing device which maintains constant positive airway pressure and method of use which analyzes an inspiratory flow waveform to titrate such a pressure value.
Obstructive sleep apnea syndrome (OSAS) is a well recognized disorder which may affect as much as 1-5% of the adult population. OSAS is one of the most common causes of excessive daytime somnolence. OSAS is most frequent in obese males, and it is the single most frequent reason for referral to sleep disorder clinics.
OSAS is associated with all conditions in which there is anatomic or functional narrowing of the patient""s upper airway, and is characterized by an intermittent obstruction of the upper airway occurring during sleep. The obstruction results in a spectrum of respiratory disturbances ranging from the total absence of airflow (apnea) to significant obstruction with or without reduced airflow (hypopnea and snoring), despite continued respiratory efforts. The morbidity of the syndrome arises from hypoxemia, hypercapnia, bradycardia and sleep disruption associated with the apneas and arousals from sleep.
The pathophysiology of OSAS is not fully worked out. However, it is now well recognized that obstruction of the upper airway during sleep is in part due to the collapsible behavior of the supraglottic segment during the negative intraluminal pressure generated by inspiratory effort. Thus, the human upper airway during sleep behaves as a Starling resistor, which is defined by the property that the flow is limited to a fixed value irrespective of the driving (inspiratory) pressure. Partial or complete airway collapse can then occur associated with the loss of airway tone which is characteristic of the onset of sleep and may be exaggerated in OSAS.
Since 1981, continuous positive airway pressure (CPAP) applied by a tight fitting nasal mask worn during sleep has evolved as the most effective treatment for this disorder, and is now the standard of care. The availability of this non-invasive form of therapy has resulted in extensive publicity for apnea and the appearance of large numbers of patients who previously may have avoided the medical establishment because of the fear of tracheostomy. Increasing the comfort of the system, which is partially determined by minimizing the necessary nasal pressure, has been a major goal of research aimed at improving patient compliance with therapy. Various systems for the treatment of obstructive sleep apnea are disclosed, for example, in xe2x80x9cReversal of Obstructive Sleep Apnea by Continuous Positive Airway Pressure Applied Through The Naresxe2x80x9d, Sullivan et al, Lancet, 1981, 1:862-865; and xe2x80x9cReversal of The xe2x80x98Pickwickian Syndromexe2x80x99 By Long-Term Use of Nocturnal Nasal-Airway Pressurexe2x80x9d; Rapaport et al., New England Journal of Medicine, Oct. 7, 1982. Similarly, the article xe2x80x9cInduction of upper airway occlusion in sleeping individuals with subatmospheric nasal pressurexe2x80x9d, Schwartz et al, Journal of Applied Physiology, 1988, 64, pp. 535-542, discusses various polysomnographic techniques. Each of these articles are hereby incorporated herein by reference.
Despite its success, limitations to the use of nasal CPAP exist. These mostly take the form of discomfort from the mask and the nasal pressure required to obliterate the apneas. Systems for minimizing the discomfort from the mask are disclosed, for example, in U.S. Pat. No. 4,655,213, Rapaport et al, and U.S. Pat. No. 5,065,756, Rapaport, as well as in xe2x80x9cTherapeutic Options For Obstructive Sleep Apneaxe2x80x9d, Garay, Respiratory Management, July/August 1987, pp. 11-15; and xe2x80x9cTechniques For Administering Nasal CPAPxe2x80x9d, Rapaport, Respiratory Management, July/August 1987, pp. 18-21 (each being hereby incorporated herein by reference). Minimizing the necessary pressure remains a goal of the preliminary testing of a patient in the sleep laboratory. However, it has been shown that this pressure varies throughout the night with sleep stage and body position. Furthermore, the therapeutic pressure may both rise or fall with time in patients with changing anatomy (nasal congestion/polyps), change in weight, changing medication or with alcohol use. Because of this, most sleep laboratories currently prescribe the setting for home use of nasal CPAP pressure based upon the single highest value of pressures needed to obliterate apneas during a night of monitoring in the sleep laboratory. Retesting is often necessary if the patient complains of incomplete resolution of daytime sleepiness, and may reveal a change in the required pressure.
The invention is therefore directed to a method and apparatus, in a system for the treatment of obstructive sleep apnea, for optimizing the controlled positive pressure to thereby minimize the flow of air from a flow generator while still ensuring that flow limitation in the patient""s airway does not occur. In particular, the invention relates to a breathing device and method of use to adjust a controlled positive pressure to the airway of a patient by detecting flow limitation from analysis of an inspiratory flow waveform.
In accordance with the invention, an apparatus for the treatment of obstructive sleep apnea is provided, comprising a source of air, and means for directing an air flow from said source to a patient. This part of the system may be of the type disclosed, for example, in U.S. Pat. No. 5,065,756. In addition, means are provided for sensing the waveform of said airflow, to detect deviations therein that correspond to flow limitation in the air supplied to the patient. Such deviations may be, for example, deviations from a substantially sinusoidal waveform, flattening, or the presence of plateaus, in the portions of the waveform corresponding to inspiration of the patient. In response to such variations in said airflow, the system of the invention increases or decreases the pressure to the patient.
In accordance with the method of the invention, the controlled positive pressure to the patient is increased in response to the detection of flow waveform portions corresponding to flow limitations in the patient airway. Such pressure increases may be effected periodically. Similarly, the controlled positive pressure may be periodically decreased in the absence of such flow limitation. The system may be provided with a program that periodically decreases the controlled positive pressure in the absence of detection of flow limitations in the patient airway, and that periodically increases the pressure in the presence of detection of such flow limitations.
The method for determining whether to increase or decrease the controlled positive pressure is comprised of several steps. The first step is to detect the presence of a valid breath and store an inspiratory waveform of that breath for further analysis. Next, the waveform of the stored breath is analyzed regarding its shape for presence of flow limitation. Whether flow limitation is present is in part determined by flow limitation parameters calculated from the shape of the waveforms of the current breath and of the immediately preceding breath. Once the presence of flow limitation has been analyzed, the system determines an action to take for adjustment of the controlled positive pressure. The pressure setting is raised, lowered or maintained depending on whether flow limitation has been detected and on the previous actions taken by the system.
The preferred breathing device or apparatus consists of a flow generator, such as a variable-speed blower, a flow sensor, an analog to digital converter, a microprocessor, and a pressure controller, such as a blower motor speed control circuit, a patient connection hose, a nasal coupling, such as a nose mask or similar fitting, and, optionally, a pressure transducer. Alternative patient circuits may be employed, such as those disclosed in U.S. Pat Nos. 4,655,213 and 5,065,756. For example, a positive pressure breathing gas source may be connected to a pressure control valve proximate the breathing gas source and connected to a nasal mask having a venting means.
In the preferred embodiment, the blower supplies air through the flow sensor to the patient via a hose and nasal coupling. The microprocessor obtains the flow waveform from the digitized output of the flow sensor. Using the method of the present invention described herein, the microprocessor adjusts the speed of the blower via the motor control circuit to change the air pressure in the patient supply hose. A pressure transducer may be provided to measure the actual pressure in the patient hose. In addition, the microprocessor may store measured pressure and flow waveform values in its data memory to provide a history for real-time or off-line processing and analysis.
Other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.